1. Field of the Invention
The present invention relates to a print data generation method, a printing apparatus, a method of generating a mask, and a mask pattern. Specifically, the present invention relates to a method of generating a mask, and a mask pattern, which are used for multi-pass printing.
2. Description of the Related Art
Printing apparatuses of these years include an apparatus which performs so-called multi-pass printing, in which a print in a given printing region is completed by multiple scanning, for the purpose of forming an image which is intended to be formed on a printing medium with a higher image quality. Generally for the multi-pass printing method, AND operation is performed between print data and a mask pattern, which determines permitting/non-permitting printing based on each print data for each unit same as a pixel of print data, and thereby print data for each scanning are generated. Descriptions will be provided below for the multi-pass printing.
FIG. 4 which will be described in conjunction with an embodiment of the present invention schematically shows a printing head and print patterns for the purpose of describing the multi-pass printing method. Reference numeral P0001 denotes a printing head. In FIG. 4, it is supposed that the printing head has 16 printing elements (nozzles) for the purpose of simplifying the following descriptions. The nozzles are divided into four nozzle groups: a first to a fourth nozzle groups, as shown in FIG. 4. Each nozzle group includes four nozzles. Reference numeral P0002 denotes mask patterns. In each mask pattern, a unit area corresponding to a pixel which permits printing based on print data is blackened, and a unit area corresponding to a pixel which does not permit printing based on print data is whitened. The patterns which are printed by those nozzle groups are complementary to one another. When these printed patterns are superposed over one another, a print to be made in a region corresponding to 4×4 areas is completed by four times of scanning.
Each of print patterns denoted by reference numerals P0003 to P0006 shows how an image is progressing toward its completion each time a scan is repeated. Every time one scan is completed, a printing medium is conveyed in a distance equal to a width of a corresponding one of the nozzle groups in a direction indicated by an arrow. Thereby, an image to be formed in a given region (corresponding to the width of each of the nozzle groups) in the printing medium is completed by print scanning four times.
As described above, printing of each region in the printing medium is completed by scanning a plurality of times and by using a plurality of printing nozzle groups. This brings about an effect of reducing variation in printing properties particular to the respective nozzles, of reducing variation in precision with which a printing medium is conveyed, and of reducing equivalent problems. In addition, if ingenuity is exerted in arranging print permitting areas and print non-permitting areas in each of the mask patterns, this can be adopted as countermeasures for other various problems with an image and with reliability of the printing apparatus.
For example, in the case of an inkjet printing head of recent years which ejects a larger number of fine ink droplets with higher frequencies, the direction in which droplets are ejected from each of nozzles located in end portions of the printing head has a tendency to be inward. In this case, dots to be formed by the nozzles located in the end portions of each nozzle row sometimes causes a white stripe (hereinafter referred to as an “end stripe”) with a pitch equal to the printing width of the printing head. This is because the dots are landed in inner positions which deviate from their normal positions. Even in such a situation, if ingenuity is exerted in arranging the foregoing mask patterns, this makes it possible to make the end stripe less conspicuous (Japanese Patent Application Laid-open No. 2002-096455).
FIG. 5 which will be similarly described in conjunction with the embodiment of the present invention shows an example of mask patters to be adopted for the purpose of reducing the occurrence of this kind of end stripe. Blackened areas in the mask patterns shown in FIG. 5 perform the same function as the blackened areas in the mask patterns shown in FIG. 4 perform, and indicate areas which permit printing based on print data (print permitting areas). On the other hand, whitened areas in the mask patterns shown in FIG. 5 perform the same function as the whitened areas in the mask patterns shown in FIG. 4 perform, and indicate areas which does not permit printing based on print data (print non-permitting areas). In FIG. 5, a case where a multi-pass printing with four passes is performed four times by use of a printing head with 768 nozzles (a print is completed by scanning four times) is shown as the example. The 768 nozzles in total of the example shown in FIG. 5 are divided into four nozzle groups similarly to the example shown in FIG. 4. In the case of the mask shown in FIG. 5, a printing ratio (a ratio of the number of blackened areas constituting the mask pattern to the sum of the numbers of blackened and whitened areas constituting the same mask pattern) varies depending on where the nozzles are located. Printing ratios of a mask pattern corresponding to the first nozzle group are 10% and 20% respectively from nozzles shown lowermost in the figure. In addition, printing ratios of a mask pattern corresponding to the second nozzle group are 30% and 40%. Printing ratios of a mask pattern corresponding to the third nozzle group are 40% and 30%. Printing ratios of a mask pattern corresponding to the fourth nozzle group are 20% and 10%. A total of the printing ratios of the mask patterns corresponding to these four nozzle groups is 100%. More specifically, mask patterns corresponding to respective lower parts of the first to fourth nozzle groups are complementary to one another and then have the printing ratio of 100% (=10%+40%+30%+20%). Also, mask patterns corresponding to respective upper parts of the first to fourth nozzle groups are complementary to one another and then have the printing ratio of 100% (=20%+40%+30%+10%).
In the case of the mask shown in FIG. 5, as described above, the printing ratios corresponding to nozzles located in the center parts are set relatively high. The closer to the end parts nozzles are located, the lower the printing ratios corresponding to the nozzles are. It has been observed that the direction in which droplets are ejected from each of nozzles located in end portions of a nozzle row has a more marked tendency to be inward when a print is made by ejecting finer ink droplets in higher density with higher frequencies. For this reason, the setting of the printing ratios in the end portions lower than the printing ratios in the center portions makes it possible to reduce the tendency that the direction in which droplets are ejected from each of the nozzles located in the end portions of each nozzle row are inward. In addition, even if the direction in which droplets are ejected from each of the nozzles located in the end portions of the nozzle row has a slight tendency to be inward, the mask as shown in FIG. 5 brings about an effect of making less conspicuous the end stripe which would otherwise be obviously caused by deviation of landing positions of ejected inks from their normal positions, as well. This is because the number of dots to be ejected therefrom is reduced.
In the case of inkjet printing apparatuses each with the importance placed on image quality of a picture, a smaller number of dots, higher density of nozzles and higher frequencies for drive are important factors for satisfying both of the image quality and the printing speed. For this reason, the mask with the ratios decreasing from the highest in the center portion of the nozzle arrangement to the lowest in the end portions thereof along the nozzle arrangement (hereinafter also referred to as a “gradation mask”) as shown in FIG. 5 is generally considered as being useful of inkjet printing apparatuses of recent years with the importance placed on high-quality printing such as image quality of a picture.
In addition, a random mask with white noise properties as described in Japanese Patent Application Laid-open No. 7-052390 (1995) and a mask with blue noise properties as described in Japanese Patent Application Laid-open No. 2002-144552 are often employed as a mask pattern used for the multi-pass printing method. These mask patterns have a characteristic that print permitting areas and print non-permitting areas are arranged a periodically. Accordingly, these mask patterns have an advantage that a fine texture rarely appears in an image formed by use of the multi-pass printing method.
If the mask patterns each with the characteristic of the a periodical arrangement as described in Japanese Patent Application Laid-open Nos. 7-052390 (1995) and 2002-144552 are applied to the mask as described in Japanese Patent Application Laid-open No. 2002-096455, this makes it possible to print an image with high-quality which meets requirements for a smaller number of dots and higher density of nozzles.
In a case, however, where the masks disclosed as the prior arts are used in combination as described above, such a use can not be sometimes adapted for high-speed printing. Specifically, in the case of the foregoing conventional mask patterns respectively with the white noise properties and the blue noise properties, and in the case of the mask pattern with printing ratios varying depending on the positions of the nozzles of the printing head, the print permitting areas often exist adjacent to one another in a direction in which the printing head scans. In the case of the gradation mask described in Japanese Patent Application Laid-open No. 2002-096455 in particular, the printing ratios of the masks corresponding to the nozzles located in the center portion are relatively high. For this reason, the print permitting areas are often adjacent to one another in the scanning direction. On the other hand, for the purpose of executing printing in the scanning direction based on print data corresponding to one of the nozzles, the frequency for driving the nozzle is often set so that the nozzle can be driven depending on the distance (pitch) between two adjacent print permitting areas in the mask. In other words, when the highest possible frequencies for driving the nozzles of the printing head are constant, it needs to be considered that the highest frequencies each for driving the nozzle are caused to correspond to a distance which is the shortest among distances between the two print permitting areas in the scanning direction (a distance between the adjacent print permitting areas in the case of the example described in the foregoing patent documents). In this case, the shorter this shortest distance is, the lower the scanning speed of the printing head needs to be, so that dots can be printed in positions, the distance between which is the shortest.
By contrast, consideration can be give to a mask in which, as shown in FIGS. 41A to 41D, for example, mask data of the mask patterns are arranged in a way that, simply, print permitting areas are not adjacent to each other, and in which the shortest distance can be thus extended. FIGS. 41A to 41D respectively show mask patterns to be used for the first to the fourth passes in the multi-pass printing method. In the case of these mask patterns, mask data are arranged in a way that the print permitting areas are not adjacent to each other in the scanning direction. This arrangement makes it possible to double the scanning speed in comparison with the case where there are print permitting areas adjacent to each other, and thus to realize high-speed printing, when the frequencies for driving the nozzles of the printing head are set to be constant.
When, however, the mask patterns each with the characteristic of the periodical arrangement as shown in FIGS. 41A to 41D are applied to such a mask pattern with the characteristic of the a periodical arrangement as described in Japanese Patent Application Laid-open Nos. 7-052390 (1995) and 2002-144552, this application impairs the characteristic of the a periodical arrangement which is brought about by the mask pattern. In other words, the foregoing periodical patterns do no have the characteristic of the a periodical arrangement which is brought by the mask patterns as described in these patent documents. This makes it impossible to expect high-quality printing which can be realized on the basis of the techniques described in these patent documents.